1. Field of the Invention
The present invention relates to a piston ring structural body for an internal combustion engine and, more particularly, relates to a piston ring structural body wherein an outer ring continuously formed of a resinous material without being provided with an abutment is pressed radially outwards by a ring tensile force applying member via an inner ring continuously formed of a resinous material without being provided with an abutment. The present invention also relates to a method of mounting the piston ring structural body.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. HEI 9-280373 discloses that a resinous piston ring continuously formed without being provided with an abutment is used to reduce the amount of blow-by gas (gas which leak to a crank case through a gap between a piston ring and a cylinder). In this publication, there is disclosed a piston ring structural body wherein an outer ring continuously formed of polytetrafluoroethylene (PTFE) as a fluororesin without being provided with an abutment is pressed radially outwards by a ring tensile force applying member composed of a metal coil expander via an inner ring also continuously formed of PTFE without being provided with an abutment.
In the piston ring structural body disclosed in the aforementioned publication, the ring is divided into the inner and outer rings. While the inner ring ensures sealability around a ring groove, the outer ring ensures sealability between a sliding portion thereof and a cylinder wall.
It is to be noted herein that the piston ring structural body disclosed in the aforementioned publication uses PTFE for both the inner and outer rings. However, PTFE has a low threshold value of PV (the product of a real pressure P and sliding speed V) and cannot provide the sliding portion that slides on the cylinder wall with sufficient durability. Furthermore, PTFE has a large coefficient of thermal expansion. Thus, when the temperature rises, the pressing force applied to the cylinder wall due to a tensile force of the coil expander becomes larger, which causes a problem of an increase in frictional loss of the engine.